Cobalt-titania catalysts, process utilizing these catalysts for the preparation of hydrocarbons from synthesis gas, and process for the preparation of said catalysts (C-2448)

ABSTRACT

A supported particulate cobalt catalyst is formed by dispersing cobalt, alone or with a metal promoter, particularly rhenium, as a thin catalytically active film upon a particulate titania or titania-containing support, especially one wherein the rutile:anatase ratio of the support is at least about 3:2. This catalyst can be used to convert an admixture of carbon monoxide and hydrogen to a distillate fuel constituted principally of an admixture of linear paraffins and olefins, particularly a C 10  + distillate, at high productivity, with low methane selectivity. A process is also disclosed for the preparation of these catalysts.

This is a division of application Ser. No. 252,215, filed Oct. 3, 1988,now U.S. Pat. No. 4,962,078, which is a continuation-in-part ofapplication Ser. No. 046,649, filed May 7, 1987, now abandoned.

BACKGROUND AND PROBLEMS

1. Field of the Invention

This invention relates to catalyst compositions, process wherein thesecompositions are used for the preparation of liquid hydrocarbons fromsynthesis gas, and process for the preparation of said catalysts. Inparticular, it relates to catalysts, and process wherein C₁₀ +distillate fuels, and other valuable products, are prepared by reactionof carbon monoxide and hydrogen over cobalt catalysts wherein the metalis dispersed as a thin film on the outside surface of a particulatetitania carrier or support.

2. The Prior Art

Particulate catalysts, as is well known, are normally formed bydispersing catalytically active metals, or the compounds thereof uponcarriers, or supports. Generally, in making catalysts the objective isto disperse the catalytically active material as uniformly as possiblethroughout a particulate porous support, this providing a uniformity ofcatalytically active sites from the center of a particle outwardly.

Catalysts have also been formed by dispersing catalytically activematerials upon dense support particles; particles impervious topenetration by the catalytically active materials Ceramic or metal coreshave been selected to provide better heat transfer characteristics,albeit generally the impervious dense cores of the catalyst particlesoverconcentrates the catalytically active sites within a reduced reactorspace and lessens the effectiveness of the catalyst. Sometimes, even informing catalysts from porous support particles greater amounts of thecatalytic materials are concentrated near the surface of the particlessimply because of the inherent difficulty of obtaining more uniformdispersions of the catalytic materials throughout the porous supportparticles. For example, a catalytic component may have such strongaffinity for the support surface that it tends to attach to the mostimmediately accessible surface and cannot be easily displaced andtransported to a more central location within the particle. Catalystdispersion aids, or agents are for this reason often used to overcomethis effect and obtain better and more uniform dispersion of thecatalytically active material throughout the catalyst particles.

Fischer-Tropsch synthesis for the production of hydrocarbons from carbonmonoxide and hydrogen is now well known, and described in the technicaland patent literature. The earlier Fischer-Tropsch catalysts wereconstituted for the most part of non-noble metals dispersed throughout aporous inorganic oxide support. The Group VIII non-noble metals, iron,cobalt, and nickel have been widely used in Fischer-Tropsch reactions,and these metals have been promoted with various other metals, andsupported in various ways on various substrates, principally alumina.Most commercial experience, however, has been based on cobalt and ironcatalysts. The first commercial Fischer-Tropsch operation utilized acobalt catalyst, though later more active iron catalysts were alsocommercialized. The cobalt and iron catalysts were formed by compositingthe metal throughout an inorganic oxide support. An important advance inFischer-Tropsch catalysts occurred with the use of nickel-thoria onkieselguhr in the early thirties. This catalyst was followed within ayear by the corresponding cobalt catalyst, 100 Co:18 ThO₂ :100kieselguhr, parts by weight, and over the next few years by catalystsconstituted of 100 Co:18 ThO₂ :200 kieselguhr and 100 Co:5 ThO₂ :8MgO:200 kieselguhr, respectively. These early cobalt catalysts, however,are of generally low activity necessitating a multiple staged process,as well as low synthesis gas throughout. The iron catalysts, on theother hand, are not really suitable for synthesis gas conversion due tothe high degree of water gas shift activity possessed by iron catalysts.Thus, more of the synthesis gas is converted to carbon dioxide inaccordance with the equation H₂ +2CO→(CH₂)_(x) +CO₂ ; with too little ofthe synthesis gas being converted to hydrocarbons and water as in themore desirable reaction, represented by the equation: 2H₂ +CO →(CH₂)_(x) +H₂ O.

U.S. Pat. No. 4,542,122 by Payne et al, which issued Sep. 17, 1985,describes improved cobalt catalyst compositions useful for thepreparation of liquid hydrocarbons from synthesis gas. These catalystcompositions are characterized, in particular, as cobalt-titania orthoria promoted cobalt-titania, wherein cobalt, or cobalt and thoria, iscomposited or dispersed upon titania, or titania-containing support,especially a high rutile content titania. U.S. Pat. No. 4,568,663 byMauldin, which issued Feb. 4, 1986, also discloses cobalt-titaniacatalysts to which rhenium is added to improve catalyst activity, andregeneration stability. These catalysts have performed admirably well inconducting Fischer-Tropsch reactions, and in contrast to earlier cobaltcatalysts provide high liquid hydrocarbon selectivities, with relativelylow methane formation.

Recent European Publication 0 178 008 (based on Application No.85201546.0, filed: 25.09.85) and European Publication 0 174 696 (basedon Application No. 852011412.5, filed: 05.09.85), having priority dates04.10.84 NL 8403021 and 13.09.84 NL 8402807, respectively, also disclosecobalt catalysts as well as a process for the preparation of suchcatalysts by immersion of a porous carrier once or repetitively within asolution containing a cobalt compound. The cobalt is dispersed over theporous carrier to satisfy the relation ΣV_(p) /ΣV_(c) ≦0.85 and ΣV_(p)/ΣV_(c) ≦0.55, respectively, where V_(c) represents the total volume ofthe catalyst particles and V_(p) the peel volumes present in thecatalyst particles, the catalyst particles being regarded as constitutedof a kernel surrounded by a peel. The kernel is further defined as oneof such shape that at every point of the kernel perimeter the shortestdistance (d) to the perimeter of the peel is the same, d being equal forall particles under consideration, and having been chosen such that thequantity of cobalt present in ΣV_(p) is 90% of the quantity of cobaltpresent in ΣV_(c). These particular catalysts, it is disclosed, showhigher C₅ + selectivities than catalysts otherwise similar except thatthe cobalt component thereof is homogeneously distributed, or uniformlydispersed, throughout the carrier. Suitable porous carriers aredisclosed as silica, alumina, or silica-alumina, and of these silica ispreferred. Zirconium, titanium, chromium and ruthenium are disclosed aspreferred of a broader group of promoters. Albeit these catalysts mayprovide better selectivities in synthesis gas conversion reactionsvis-a-vis catalysts otherwise similar except the cobalt is uniformlydispersed throughout the carrier, like other cobalt catalysts disclosedin the prior art, the intrinsic activities of these catalysts are toolow as a consequence of which higher temperatures are required to obtaina productivity which is desirable for commercial operations. Highertemperature operation however leads to a corresponding increase in themethane selectivity and a decrease in the production of the morevaluable liquid hydrocarbons.

Productivity, which is defined as the standard volumes of carbonmonoxide converted/volume catalyst/hour, is, of course, the life bloodof a commercial operation. High productivities are essential inachieving commercially viable operations. However, it is also essentialthat high productivity be achieved without high methane formation, formethane production results in lower production of liquid hydrocarbons.Accordingly, an important and necessary objective in the production anddevelopment of catalysts is to produce catalysts which are capable ofhigh productivity, combined with low methane selectivity.

Despite improvements, there nonetheless remains a need for catalystscapable of increased productivity, without increased methaneselectivity. There is, in particular, a need to provide further improvedcatalysts, and process for the use of these catalysts in synthesis gasconversion reactions, to provide further increased liquid hydrocarbonselectivity, especially C₁₀ + liquid hydrocarbon selectivity, withfurther reduced methane formation.

3. Objects

It is, accordingly, the primary objective of this invention to fill thisand other needs.

It is, in particular, an object of this invention to provide furtherimproved, novel supported cobalt catalyst compositions, and processutilizing such compositions for the conversion of synthesis gas at highproductivity, and low methane selectivity, to high quality distillatefuels characterized generally as C₁₀ + linear paraffins and olefins.

A further and more particular object is to provide novel, supportedcobalt catalyst compositions, both promoted and unpromoted whichapproach, or meet the activity, selectivity and productivity of powderedcatalysts but yet are of a size acceptable for commercial synthesis gasconversion operation.

A further object is to provide a process utilizing such catalystcompositions for the production from synthesis gas to C₁₀ + linearparaffins and olefins, at high productivity with decreased methaneselectivity.

Yet another object is to provide a process for the preparation of suchcatalysts.

4. The Invention

These objects and others are achieved in accordance with this inventionembodying a supported particulate cobalt catalyst formed by dispersingthe cobalt as a thin catalytically active film upon the surface of aparticulate titania support or substantially titania containing supportand preferably wherein the rutile:anatase ratio of the titania is atleast about 3:2. This catalyst can be used to produce, by contact andreaction at reaction conditions with an admixture of carbon monoxide andhydrogen, a distillate fuel constituted principally of an admixture oflinear paraffins and olefins, particularly a C₁₀ + distillate, at highproductivity, with low methane selectivity. This product can be furtherrefined and upgraded to high quality fuels, and other products such asmogas, diesel fuel and jet fuel, especially premium middle distillatefuels of carbon numbers ranging from about C₁₀ to about C₂₀.

In accordance with this invention the catalytically active cobaltcomponent is dispersed and supported upon the titania particles as athin catalytically active surface layer, or film, ranging in averagethickness from about 0.02 millimeters (mm) to about 0.20 mm, preferablyfrom about 0.04 mm to about 0.20 mm, with the loading of the cobaltbeing sufficient to achieve the productivity required for viablecommercial operations, e.g., a productivity in excess of about 150. Thecobalt loading, expressed as the weight metallic cobalt per packed bulkvolume of catalyst, that achieves this result is at least about 0.04grams (g) per cubic centimeter (cc) and preferably at least about 0.05g/cc. Higher levels of cobalt tend to increase the productivity furtherand an upper limit of cobalt loading is a function of cobalt cost,diminishing increases in productivity with increases in cobalt, and easeof depositing cobalt. A suitable range may be from about 0.04 g/cc toabout 0.15 g/cc, preferably about 0.05 g/cc to about 0.15 g/cc morepreferably about 0.05 g/cc to about 0.09 g/cc. The feature of a highcobalt metal loading in a thin catalytically active layer located at thesurface of the particles is essential in optimizing the activity,selectivity and productivity of the catalyst in producing liquidhydrocarbons from synthesis gas, while minimizing methane formation.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE contains a plot of methane yield versus productivity forvarious catalysts.

Metals such as rhenium, zirconium, hafnium, cerium, thorium and uranium,or the compounds thereof, can be added to cobalt to increase theactivity and regenerability of the catalyst. Thus, the thincatalytically active layers, or films, formed on the surface of thetitania or substantially titania-containing support particles, caninclude in addition to a catalytically active amount of cobalt, any oneor more of rhenium, zirconium, hafnium, cerium, uranium, and thorium, oradmixtures of these with other metals or compounds thereof. Preferredthin catalytically active layers, or films, supported on the titania ortitania-containing support, thus include cobalt-rhenium,cobalt-zirconium, cobalt-hafnium, cobalt-cerium, cobalt-uranium, andcobalt-thorium, with or without the additional presence of other metalsor compounds thereof.

A particularly preferred catalyst is one wherein cobalt, or cobalt and apromoter, is dispersed as a thin catalytically active film upon acarrier or support that is titania, TiO₂, or a material that issubstantially titania, and preferably at least 50% titania, and stillmore preferably at least 80% titania, in which the titania has arutile:anatase weight ratio of at least about 3:2, as determined by ASTMD 3720-78: Standard Test Method for Ratio of Anatase to Rutile InTitanium Dioxide Pigments By Use of X-ray Diffraction. Generally, thecatalyst is one wherein the titania has a rutile:anatase ratio rangingat least about 3:2 to about 100:1, or greater, and more preferably fromabout 4:1 to about 100:1, or greater. Where any one of rhenium,zirconium, hafnium, cerium, thorium, or uranium metals, respectively, isadded to the cobalt as a promoter to form the thin catalytically activefilm, the metal is added to the cobalt in concentration sufficient toprovide a weight ratio of cobalt:metal promoter ranging from about 30:1to about 2:1, preferably from about 20:1 to about 5:1 Rhenium andhafnium are the preferred promoter metals, rhenium being more effectivein promoting improved activity maintenance on an absolute basis, withhafnium being more effective on a cost-effectiveness basis. Thesecatalyst compositions, it has been found, produce at high productivity,with low methane selectivity, a product which is predominately C₁₀ +linear paraffins and olefins, with very little oxygenates. Thesecatalysts also provide high activity, high selectivity and high activitymaintenance in the conversion of carbon monoxide and hydrogen todistillate fuels.

The cobalt catalysts of this invention, as contrasted with (i) cobaltcatalysts, the cobalt portion of which is uniformly distributedthroughout the support particles or (ii) cobalt catalysts having arelatively thick surface layer of cobalt on the support particles, haveproven especially useful for the preparation of liquid hydrocarbons fromsynthesis gas at high productivities, with low methane formation. Incontrast with the catalysts of this invention, the prior art catalystsare found to have lower activity, and especially poorer selectivity dueto a severe diffusion limitation. These catalysts (i) and (ii), supra,at high productivities, produce altogether too much methane. Asproductivity is increased to produce greater conversion of the carbonmonoxide to hydrocarbons, increased amounts of methane are concurrentlyproduced. Thus, increased productivity with these catalysts could onlybe obtained at the cost of increased methane formation. This resultoccurs, it is believed, because the carbon monoxide and hydrogenreactants all too slowly diffuse through the pores of the particulatecatalyst which becomes filled with a liquid product, thus resulting inunderutilization of the catalytically active sites located within theinterior of the particles Both hydrogen and carbon monoxide must diffusethrough the product-liquid filled pores, but hydrogen diffuses throughthe pores at a greater rate of speed than the carbon monoxide. Sinceboth the hydrogen and the carbon monoxide are reacting at the catalyticsites at an equivalent rate, a high H₂ /CO ratio is created in theinterior of the particle which leads to high methane formation. As therate of reaction is increased, e.g., by incorporating higher intrinsicactivity or by operating at higher temperature, the catalyst becomesmore limited by the rate of diffusion of the reactants through thepores. Selectivities are especially poor under the conditions of highproductivity. Thus, the catalyst used during a Fischer-Tropschhydrocarbon synthesis reaction is one the pores of which become filledwith the product liquid. When the CO and H₂ are passed over the bed ofcatalyst and consumed at a rate which is faster then the rate ofdiffusion, H₂ progresses to the interior of the particle to a muchgreater extent than the CO, leaving the interior of the particles richin H₂, and deficient in CO. The formation of methane within the particleinterior is thus favored due to the abnormally high H.sub. 2 /CO ratio;an unfavorable result since CH₄ is not a desirable product. The extentto which selectivity is debited depends on the magnitude of thedifference between the rate of diffusion and the rate of reaction, i.e.,the productivity.

The catalyst of this invention is thus one wherein essentially all ofthe active cobalt is deposited on the surface of the titania ortitania-containing support particles. The surface film of cobalt must bevery thin and contain an adequate loading of cobalt to maximize reactionof the hydrogen and carbon monoxide at the surface of the catalyticparticle. The surface film of cobalt as stated thus ranges generallyfrom about 0.02 mm to about 0.20 mm, preferably from about 0.04 mm toabout 0.20 mm, with cobalt loadings at least about 0.04 g/cc, preferablyat least about 0.05 g/cc, more preferably ranging from about 0.04 g/ccto about 0.15 g/cc, still more preferably from about 0.05 g/cc to about0.15 g/cc, and even more preferably ranging from about 0.05 g/cc toabout 0.09 g/cc, calculated as metallic cobalt per packed bulk volume ofcatalyst. The promoter metal to be effective must also be containedwithin the surface film of cobalt. If extended into the interior of theparticle outside the cobalt film the promoter metal will have littlepromotional effect, if any. The metal promoter should thus also beconcentrated within the cobalt film at the surface of the catalyst, withthe weight ratio of cobalt:metal promoter, as suggested, ranging fromabout 30:1 to about 2:1, preferably from about 20:1 to about 5:1. Thethickness of the surface metal film can be conveniently measured by anElectron Probe Analyzer, e.g., one such as produced by the JEOL Company,Model No. JXA-50A. Cross-sections of the catalyst particles of thisinvention measured via use of this instrument show very high peaks, orshoulders, at the edges of the particle across the line of sweeprepresentative of cobalt concentration, with little or no cobalt showingwithin the particle interior. The edge, or "rim" of the "radiallyimpregnated catalyst" will thus contain essentially all of the cobaltadded to the catalyst. The thickness of the film, or rim, is unrelatedto the absolute size, or shape of the support particles. Virtually anysize particle can be employed as is normally employed to effect catalystreactions of this type, the diameter of the particle ranging generallyfrom about 0.5 mm to about 2 mm. The particles can be of virtually anyshape, e.g., as is normally employed to effect reactions of this type,viz., as beads or spheres, extrudates, saddles or the like. Byconcentrating the catalytic metal, or metals, on the extreme outersurface of the particles, the normal diffusion limitation of thecatalyst can be minimized to the extent that diffusion limitation is nolonger a deleterious problem. This new catalyst is more active in itsfunction of bringing about a reaction between the CO and H₂. Thecatalyst because of its having the thin layer of catalytically activemetal on its surface is in effect found to behave more ideally,approaching, in fact, the behavior of a powdered catalyst which does notexhibit diffusion limitations. However, unlike the use of powderedcatalysts, the flow of the reactants through the catalyst bed isvirtually unimpeded. Higher productivity, with lower methaneselectivity, is the result; a result of considerable commercialconsequence. At productivities (at 200° C.) greater than 150 hour⁻¹(standard volumes of carbon monoxide converted per volume of catalystper hour), notably from about 150 hour⁻¹ to about 200 hour⁻¹, less than10 mole percent of the carbon monoxide converted is converted tomethane.

In conducting synthesis gas reactions the total pressure upon the CO andH₂ reaction mixture is generally maintained above about 80 psig, andpreferably above about 140 psig. It is generally desirable to employcarbon monoxide, and hydrogen, in molar ratio of H₂ :CO above about0.5:1 and preferably equal to or above about 1.7:1 to increase theconcentration of C₁₀ + hydrocarbons in the product. Suitably, the H₂ :COmolar ratio ranges from about 0.5:1 to about 4:1, and preferably thecarbon monoxide and hydrogen are employed in molar ratio H₂ :CO rangingfrom about 1.7:1 to about 2.5:1. In general, the reaction is carried outat gas hourly space velocities ranging from about 100 V/Hr/V to about5000 V/Hr/V, preferably from about 300 V/Hr/V to about 1500 V/Hr/V,measured as standard volumes of the gaseous mixture of carbon monoxideand hydrogen (0° C., 1 Atm.) per hour per volume of catalyst. Thereaction is conducted at temperatures ranging from about 160° C. toabout 290° C., preferably from about 190° C. to about 260° C. Pressurespreferably range from abut 80 psig to about 600 psig, more preferablyfrom about 140 psig to about 400 psig. The product generally andpreferably contains 60 percent, or greater, and more preferably 75percent, or greater, C₁₀ + liquid hydrocarbons which boil above 160° C.(320° F.)

The catalysts employed in the practice of this invention can be preparedby spray techniques where a solution of a cobalt compound, alone or inadmixture with a promoter metal compound, or compounds as a spray isrepetitively contacted with hot titania, or titania-containing supportparticles. The particulate late titania or titania-containing supportparticles are preheated to temperatures equal to or above about 140° C.and then contacted with the spray. Suitably the temperature of thetitania, or titania-containing support, ranges from about 140° C. up tothe decomposition temperature of the cobalt compound, or compounds inadmixture therewith; preferably from about 140° C. to about 190° C. Thecobalt compound employed in the solution can be any organometallic orinorganic compound which decomposes to give cobalt oxide upon initialcontact or upon calcination, such as cobalt nitrate, cobalt acetate,cobalt acetylacetonate, cobalt naphthenate, cobalt carbonyl, or thelike. Cobalt nitrate is especially preferred while cobalt halide andsulfate salts should generally be avoided. The cobalt salts may bedissolved in a suitable solvent, e.g., water, organic or hydrocarbonsolvent such as acetone, methanol, pentane or the like. The total amountof impregnation solution used should be sufficient to supply the propercatalyst loading, with the film being built up by repetitive contactsbetween the support and the solution. The preferred catalyst is onewhich consists essentially of cobalt, or cobalt and promoter, dispersedupon the titania, or titania-containing support, especially a rutilesupport. Suitably, the hot titania support is contacted with a spraywhich contains from about 0.05 g/ml to about 0.25 g/ml, preferably fromabout 0.10 g/ml to about 0.20 g/ml, of the cobalt compound or cobaltcompound plus the compound containing the promoter metal, generally fromat least about 3 to about 12 contacts, preferably from about 5 to about8 contacts, with intervening drying and calcination steps being requiredto form surface films of the required thicknesses. The hot titania, ortitania-containing support, in other words, is spray-contacted in afirst cycle which includes the spray contact per se with subsequentdrying and calcination, a second cycle which includes the spray contactper se with subsequent drying and calcination, a third spray contactwhich includes the spray contact per se with subsequent drying andcalcination, etc. to form a film of the required thickness andcomposition. The drying steps are generally conducted at temperaturesranging above about 20° C., preferably from about 20° C. to about 125°C., and the calcination steps at temperatures ranging above about 150°C., preferably from about 150° C. to about 300° C.

Titania is used as a support, either alone or in combination with othermaterials for forming a support, but the titania preferably makes up atleast about 50% of the support. The titania used for the support ispreferably one which contains a rutile:anatase ratio of at least about3:2, as determined by x-ray diffraction (ASTM D 3720-78). The titaniapreferably has a rutile:anatase ratio of from about 3:2 to about 100:1,or greater, more preferably from about 4:1 to about 100:1, or greater.The surface area of such forms of titania are less than about 50 m² /g.These weight concentrations of rutile provide generally optimumactivity, and C₁₀ + hydrocarbon selectivity without significant gas andCO₂ make.

The prepared catalyst as a final step is dried by heating at atemperature above about 20° C., preferably between 20° C. and 125° C.,in the presence of nitrogen or oxygen, or both, in an air stream orunder vacuum. It is necessary to activate the catalyst prior to use.Preferably, the catalyst is contacted with oxygen, air, or otheroxygen-containing gas at temperature sufficient to oxidize the cobaltand convert the cobalt to Co₃ O₄. Temperatures ranging above about 150°C., and preferably above about 200° C. are satisfactory to convert thecobalt to the oxide, but temperatures above about 500° C. are to beavoided unless necessary for regeneration of a severely deactivatedcatalyst. Suitably, the oxidation of the cobalt is achieved attemperatures ranging from about 150° C. to about 300° C. The metal, ormetals, contained on the catalyst are then reduced. Reduction isperformed by contact of the catalyst, whether or not previouslyoxidized, with a reducing gas, suitably with hydrogen or ahydrogen-containing gas stream at temperatures above about 200° C.;preferably above about 250° C. Suitably, the catalyst is reduced attemperatures ranging from about 200° C. to about 500° C. for periodsranging from about 0.5 to about 24 hours at pressures ranging fromambient to about 40 atmospheres. A gas containing hydrogen and inertcomponents in admixture is satisfactory for use in carrying out thereduction.

The catalysts of this invention can be regenerated, and reactivated torestore their initial activity and selectivity after use by washing thecatalyst with a hydrocarbon solvent, or by stripping with a gas.Preferably the catalyst is stripped with a gas, most preferably withhydrogen, or a gas which is inert or non-reactive at strippingconditions such as nitrogen, carbon dioxide, or methane. The strippingremoves the hydrocarbons which are liquid at reaction conditions. Gasstripping can be performed at substantially the same temperatures andpressures at which the reaction is carried out. Pressures can be lowerhowever, as low as atmospheric or even a vacuum. Temperatures can thusrange from about 160° C. to about 290° C., preferably from about 190° C.to about 260° C., and pressures from below atmospheric to about 600psig, preferably from about 140 psig to about 400 psig. If it isnecessary to remove coke from the catalyst, the catalyst can becontacted with a dilute oxygen-containing gas and the coke burned fromthe catalyst at controlled temperature below the sintering temperatureof the catalyst. Most of the coke can be readily removed in this way.The catalyst is then reactivated, reduced, and made ready for use bytreatment with hydrogen or hydrogen-containing gas as with a freshcatalyst.

The invention will be more fully understood by reference to thefollowing examples and demonstrations which present comparative dataillustrating its more salient features.

The catalysts of this invention are disclosed in the following examplesand demonstrations as Catalysts Nos. 14-21. These are catalysts whichhave surface films falling within the required range of thicknesses, andthe surface film contains the required cobalt metal loadings. It will beobserved that all of Catalysts Nos. 14-21 were formed by a processwherein a heated particulate TiO₂ substrate was repetitively contactedwith a dilute spray solution containing both the cobalt and rheniumwhich was deposited as a thin surface layer, or film, upon theparticles. Catalysts Nos. 14-21 are contrasted in a series of synthesisgas conversion runs with Catalysts Nos. 1-8, catalysts wherein themetals are uniformly dispersed throughout the TiO₂ support particles.They are also contrasted in runs made with Catalysts Nos. 9-13, also"rim" catalysts but catalysts wherein the surface films, or rims, areeither too thick (Catalysts Nos. 11-13), however prepared, or do notcontain an adequate cobalt metal loading within the surface film, or rim(Catalyst Nos. 9 and 10). From the data presented, at highproductivities the catalysts formed from the uniformly impregnated TiO₂spheres produce high methane. Moreover, even wherein a film of thecatalytic metal is formed on the surface of the particles, it isessential that the surface film, or rim of cobalt be very thin and alsocontain an adequate loading of cobalt in the film. This is necessary tomaximize reaction of the H₂ and CO at the surface of the particlewherein the cobalt metal reaction sites are located, whilesimultaneously reactions within the catalyst but outside the metal filmor rim are suppressed to maximize productivity, and lower methaneselectivity. The following data thus show that the catalysts of thisinvention, i.e, Catalysts Nos. 14-21, can be employed at productivitiesabove 150 hour⁻¹ to 200 hour⁻¹, and greater, to produce no more and evenless methane than is produced by (i) catalysts otherwise similar exceptthat the catalysts contain a thicker surface film, i.e., Catalysts Nos.11-13, or (ii) catalysts which contain an insufficient cobalt metalloading within a surface film of otherwise acceptable thinness, i.e.,Catalyst Nos. 9 and 10. The data show that the catalysts of thisinvention at productivities ranging above about 150 hour⁻¹ to about 200hour⁻¹, and greater, can be employed to produce liquid hydrocarbons atmethane levels well below 10 mole percent.

EXAMPLES 1-8

A series of twenty-one different catalysts were prepared from titania,TiO₂, supplied by a catalyst manufacturer in spherical form; the TiO₂having the following physical properties, to wit:

14-20 Tyler mesh size (1 mm average diameter)

86-95% rutile content (by ASTM D 3720-78 test)

14-17 m² /g BET surface area

0.11-0.16 g/cc pore volume (by mercury intrusion)

In the catalyst preparations, portions of the TiO₂ spheres wereimpregnated with cobalt nitrate and perrhenic acid via severalimpregnation techniques as subsequently described. In each instance,after drying in vacuo at 125°-185° C., the catalysts were calcined inflowing air at 250°-500° C. for 3 hours. A first series of catalysts(Catalyst Nos. 1-8) were prepared wherein the TiO₂ spheres wereuniformly impregnated, and these catalysts then used in a series of baseruns (Table 1). Catalyst Nos. 9-11 (Table 2) and 12-21 (Table 3) wereprepared such that the metals were deposited on the outside surface ofthe spheres to provide a shell, film or rim. The thicknesses of thecatalyst rim, or outer shell, were determined in each instance byElectron Microprobe Analysis. Runs were made with these catalysts, eachbeing contacted with synthesis gas at similar conditions and comparisonsthen made with those employed to provide the base runs.

Catalysts Nos 1-8, described in Table 1, were prepared as uniformlyimpregnated catalysts to wit: A series of uniformly impregnated TiO₂spheres were prepared by immersing the TiO₂ spheres in acetone solutionsof cobalt nitrate and perrhenic acid, evaporating off the solutions, andthen drying and calcining the impregnated spheres. The Co and Reloadings, expressed as gms metal per cc of catalyst on a bulk, drybasis, deposited upon each of the catalysts are given in the second andthird columns of Table 1.

Catalysts Nos. 9-11 were prepared to contain an outer rim or shell.These catalysts were prepared by a liquid displacement method whichinvolves first soaking the TiO₂ in a water-immiscible liquid, drainingoff the excess liquid, and then dipping the wet spheres into aconcentrated aqueous solution of cobalt nitrate (0.24 g Co/ml) andperrhenic acid (0.02 g Re/ml). Contact with the metal salt solution islimited to a very short period of time, during which the solutiondisplaces the pre-soak liquid from the outer surface of the supportparticles. The rim-impregnated catalyst is quickly blotted on papertowels and dried in a vacuum oven at 140° C. Results are summarized inTable 2. The second column of Table 2 thus identifies the presoakliquid, the third column the displacement time in minutes, the fourthand fifth columns the g Co/cc and g Re/cc, respectively, and the sixthcolumn the rim thickness or thickness of the outer metal shell inmicrons.

Catalysts Nos. 12-21, described in Table 3, were prepared to have metalshells or rims by use of a series of spray techniques. TiO₂ spheres werespread out on a wire screen and preheated in a vacuum oven at varioustemperatures. The hot spheres were removed from the oven, sprayed with asmall amount of metal salt solution, and returned without delay to theoven where drying and partial decomposition of the cobalt nitrate saltoccurred. The spraying sequence was repeated several times in order toimpregnate a thin outer layer or rim of Co-Re onto the support.Preparative details are as follows:

Three solutions, I, II and III, each constituted of a different solvent,and having specific concentrations of cobalt nitrate and perrhenic acidwere employed in a series of spraying procedures. The three solutionsare constituted as follows:

    ______________________________________                                                Cobalt Nitrate                                                                            Perrhenic Acid                                            Solution                                                                              Concentration                                                                             Concentration                                             Number  g Co/ml     g Re/ml      Solvent                                      ______________________________________                                        I       0.12        0.01         20% H.sub.2 O                                                                 80% Acetone                                  II      0.12        0.03         H.sub.2 O                                    III     0.12        0.01         Acetone                                      ______________________________________                                    

Five separate procedures, Procedures A, B, C, D and E, respectively,employing each of these three solutions, were employed to preparecatalysts, as follows:

A: 30 ml of Solution I added to 50 g TiO₂ spheres in 5 sprayings

B: 30 ml of Solution I added to 50 g TiO₂ spheres in 3 sprayings

C: 25 ml of Solution I added to 50 g TiO₂ spheres in 5 sprayings

D: 50 ml of Solution II added to 100 g TiO₂ spheres in 5 sprayings

E: 25 ml of Solution III added to 50 g TiO₂ spheres in 5 sprayings

Reference is made to Table 3. The procedure employed in spray coatingthe respective catalyst is identified in the second column of saidtable, and the TiO₂ pre-heat temperature is given in the third column ofsaid table. The g Co/cc and g Re/cc of each catalyst is given in Columns4 and 5, respectively, and the thickness of the catalyst rim is given inmicrons in the sixth column of the table. (Rim thickness can be referredto in microns or in mm, for example, 0.20 mm equals 200 microns.)

The catalysts were diluted, in each instance, with equal volumes of TiO₂spheres to minimize temperature gradients, and the catalyst mixture thencharged into a small fixed bed reactor unit. In preparation forconducting a run, the catalysts were activated by reduction withhydrogen at 450° C., at atmospheric pressure for one hour. Synthesis gaswith a composition of 64% H₂ -32% CO-4% Ne was then converted over theactivated catalyst at 200° C., 280 psig for a test period of at least 20hours. Gas hourly space velocities (GHSV) as given in each of thetables, represent the flow rate at 22° C. and atmospheric pressurepassed over the volume of catalyst, excluding the diluent. Samples ofthe exit gas were periodically analyzed by gas chromatography todetermine the extent of CO conversion and the selectivity to methane,expressed as the moles of CH₄ formed per 100 moles of CO converted.Selectivity to C₄ -- expressed as the wt % of C₄ -- in the hydrocarbonproduct, was calculated from the methane selectivity data using anempirical correlation developed from data obtained in a small pilotplant. A productivity FIGURE is also given for runs made with each ofthese catalysts, productivity being defined as the product of the valuesrepresented by the space velocity, the CO fraction in the feed and thefraction of the CO converted; the productivity being the volume COmeasured at 22° C. and atmospheric pressure converted per hour pervolume of catalyst.

                  TABLE 1                                                         ______________________________________                                        UNIFORMLY IMPREGNATED CATALYSTS, AND                                          GAS CONVERSION RUNS MADE THEREWITH                                            Cata-                                                                         lyst                              Pro-  Mol  Wt                               Num-  g       g             % Co  duc-  %    %                                ber   Co/cc   Re/cc   GHSV  Conv. tivity                                                                              CH.sub.4                                                                           C.sub.4 --                       ______________________________________                                        1     0.0392  0.0034  200   67     43    5.4  9.4                             2     0.0617  0.0046  750   50    120   10.5 16.9                             3     0.1003  0.0080  500   80    128   11.1 17.7                             4     0.0743  0.0056  750   64    154   11.5 18.3                             5     0.0796  0.0050  750   71    170   13.1 20.7                             6     0.1014  0.0084  750   77    185   13.9 21.8                             7     0.0925  0.0066  750   77    185   13.3 20.9                             8     0.1025  0.0068  1000  65    208   14.7 23.0                             ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    RIM CATALYSTS PREPARED BY LIQUID DISPLACEMENT METHOD,                         AND GAS CONVERSION RUNS MADE THEREWITH                                                        Displacement     Rim                                          Catalyst                                                                           Pre soak   Time             Thickness % CO Produc-                                                                            Mol.                                                                               Wt. %               Number                                                                             Liquid     Mins.  g Co/cc                                                                            g Re/cc                                                                            Microns                                                                             GHSV                                                                              Conv.                                                                              tivity                                                                             CH.sub.4                                                                           C.sub.4             __________________________________________________________________________                                                              --                   9   98% Mesitylene/                                                                          2      0.0264                                                                             0.0023                                                                             140.sup.(1)                                                                         250 83    66  6.5  11.0                     2% n-Heptanol                                                            10   98% Mesitylene/                                                                          1      0.0373                                                                             0.0031                                                                             200.sup.(1)                                                                         500 66   106  7.6  12.6                     2%-2-Ethyl-1-hexanol                                                     11   98% Mesitylene/                                                                          2      0.0459                                                                             0.0038                                                                             320  .sup.                                                                          500 70   112  9.3  15.1                     2%-2-Ethyl-1-hexanol                                                     __________________________________________________________________________     Note .sup.(1) The rim thickness of these catalysts falls within the           acceptable range, however, there is insufficient concentration of cobalt      deposited in the rim of the TiO.sub.2 spheres.                           

                                      TABLE 3                                     __________________________________________________________________________    RIM CATALYSTS PREPARED BY SPRAYING METHOD,                                    AND GAS CONVERSION RUNS MADE THEREWITH                                                   TiO.sub.2       Rim                                                Catalyst   Pre-Heat        Thickness % CO       Mol. %                                                                             Wt. %                    Number                                                                             Procedure                                                                           Temp. °C.                                                                    g Co/cc                                                                            g Re/cc                                                                            Microns                                                                             GHSV                                                                              Conv.                                                                             Productivity                                                                         CH.sub.4                                                                           C.sub.4 --               __________________________________________________________________________    12   A     140   0.0624                                                                             0.0050                                                                             250   400 85  109    8.9  14.5                     13   B     125   0.0818                                                                             0.0068                                                                             350   750 85  204    11.8 18.8                     14   C     140   0.0531                                                                             0.0045                                                                             140   800 68  174    8.2  13.5                     15   C     140   0.0613                                                                             0.0050                                                                             150   800 71  182    8.2  13.5                     16   C     140   0.0739                                                                             0.0070                                                                             130   800 81  207    8.7  14.2                     17   D     185   0.0507                                                                             0.0125                                                                             160   800 68  174    9.2  15.0                     18   E     185   0.0549                                                                             0.0049                                                                              90   800 68  174    6.5  11.0                     19   C     185   0.0483                                                                             0.0043                                                                              70   800 64  164    6.7  11.3                     20   C     185   0.0474                                                                             0.0033                                                                              90   800 65  166    7.5  12.5                     21   C     185   0.0603                                                                             0.0046                                                                              60   800 74  189    7.2  12.0                     __________________________________________________________________________

The effectiveness of these catalysts for conducting synthesis gasreactions is best illustrated by comparison of the methane selectivityat given productivity with Catalysts 1-8 (Table 1), the catalysts formedby the uniform impregnation of the metals throughout the TiO₂ catalystspheres, and Catalysts 9-11 (Table 2) and 12-21 (Table 3), thosecatalysts wherein the metals were deposited as a shell, or rim, upon theoutside of the TiO₂ catalyst spheres. The same type of comparison isthen made between certain of the latter class of catalysts, and others,which also differ one from another dependent upon the thickness of themetals-containing rim. These data are best graphically illustrated forready, visual comparison. Reference is thus made to the FIGURE whereinthe methane selectivity produced at given productivity is plotted foreach of the twenty-one catalysts described by reference to Tables 1-3. Asolid black data point is plotted for each of Catalysts Nos. 1-8, formedfrom the uniformly impregnated TiO₂ spheres, and each data point isidentified by catalyst number. An open circle is plotted for each datapoint representative of Catalysts Nos. 9-21, each is identified bycatalyst number, and the rim thickness of the catalyst is given. Thebehavior of many of these catalysts (i.e., Catalysts 9-13), it will beobserved is somewhat analogous to that of Catalysts Nos. 1-8. CatalystsNos. 14-21, however, behave quite differently from either of the othergroups of catalysts, i.e., Catalysts Nos. 1-8 or Catalysts 9-13. Themethane selectivity is thus relatively low for Catalysts Nos. 9-12, butat the same time the productivities of these catalysts are quite low. Onthe other hand, the productivities of Catalysts Nos. 2-8 tend to behigher than those of Catalysts Nos. 9-12, but at the same time thesecatalysts produce copious amounts of methane. Catalyst No. 1 shows theexpected low methane and low productivity of lightly loaded, homogeneouscatalyst. Catalyst No. 13 shows a high productivity but also highmethane selectivity and performs similarly to a homogeneous catalystwith high cobalt loadings. In striking contrast to either of thesegroups of catalysts, Catalysts Nos. 14-21, all of which fall within the"box" depicted on the FIGURE, provide very high productivities and, atthe same time, low methane selectivities. Catalysts Nos. 14-21 thusdiffer profoundly from any of Catalysts Nos. 1-13 in their behavior, andin that the metals components of these catalysts are packed into a verythin rim, or shell, on the surface of the TiO₂ support.

These data thus show that at constant temperature as productivityincreases so too does methane selectivity for both the groups ofcatalysts represented by Catalysts Nos. 1-8, the uniformly impregnatedcatalysts, and Catalysts Nos. 11-13 which have relatively thick outershells, or rims. Thus, methane selectivity increases in proportion tothe metals loadings when the metals are dispersed throughout thesupport, or carrier portion of the catalyst. Methane selectivity alsoincreases in proportion to the thickness of the catalyst rim. Albeit themethane selectivities obtained with Catalysts Nos. 9 and 10 are withinacceptable ranges, the productivities obtained with these catalysts arequite low. Catalyst No. 13 is poor on methane selectivity. Catalyst No.9, although it has a thin metallic rim and provides low methaneselectivity, its productivity is quite poor because of an insufficientloading of metals within the rim. Catalysts Nos. 14-21 which have thinmetallic rims and relatively high metals loadings within the rims, onthe other hand, provide low methane selectivities and highproductivities.

The results observed with Catalysts No. 1-8 and 11-13 are consistentwith the onset of diffusion limitation at the higher productivities,which intensifies as the catalysts become more active. In sharpcontrast, however, catalysts which have cobalt rim thicknesses of about200 microns, and less, notably from about 60 microns to about 200microns, can produce at high productivities (i.e., about 150 hr⁻¹, oreven 200 hr⁻¹), very low methane selectivities. Catalysts with very thinrims counteract the diffusion problem by limiting reaction to the outersurface of the catalyst wherein lies the catalytically active metalcomponents. The catalysts of this invention thus provide a means ofoperating at high productivity levels with low methane selectivities.Methane selectivities are reduced at higher and higher productivities,as the rim thickness is made smaller and smaller. When productivity isincreased beyond 150 hr⁻¹, the metals rim should be no more than about200 microns thick, and perhaps, even thinner. This region of operation,the best balance between activity and selectivity, is represented in theFIGURE by the area enclosed within the box formed by the dashed lines.

These data further show that the catalysts of this invention (CatalystNos. 14-21) can be readily prepared by the process of sequentially, orrepetitively spraying hot, or preheated TiO₂ spheres with solutionscontaining compounds, or salts of the metals. Suitably, the TiO₂substrate is preheated to temperatures of at least about 140° C.,suitably to temperatures ranging from abou t140° C. to about 185° C.,prior to or at the time of contact thereof with the solution. Highertemperatures can be employed, but temperatures below about 140° C. donot produce a sufficiently thin rim of the metals on the catalystsupport. The repetitive spraying technique is shown to be superior toliquid displacement technique used to prepare Catalyst Nos. 9-11 whereinonly low cobalt loadings were deposited in a single contact because ofthe cobalt concentration limit in the displacing solution. Longerdisplacement time increases the metal loading but produces a thicker rimas shown by Catalyst No. 11 compared with Catalyst No. 10. The spraytechnique provides especially good dispersion of the metals as a thinrim at the outer surface of the support particles by application of themetals a little at a time by multiple impregnations. Loading the metalsonto the catalysts in this manner increases the activity of thecatalysts, and provides higher productivity.

These reactions can be conducted with these catalysts in fixed bed, orebullating bed reactors with or without the recycle of any unconvertedgas and/or liquid product. The C₁₀ + product that is obtained is anadmixture of linear paraffins and olefins which can be further refinedand upgraded to high quality middle distillate fuels, or such otherproducts as mogas, diesel fuel, jet fuel and the like. A premium grademiddle distillate fuel of carbon number ranging from about C₁₀ to aboutC₂₀ can also be produced from the C₁₀ + hydrocarbon product.

It is apparent that various modifications and changes can be madewithout departing the spirit and scope of the present invention.

What is claimed is:
 1. A process useful for the conversion of synthesisgas to liquid hydrocarbons, which comprises contacting at reactionconditions a feed comprising carbon monoxide and hydrogen, in H₂ :COmolar ratio equal to or greater than about 0.5:1 at total pressure equalto or greater than about 80 psig, over a catalyst composition whichcomprises cobalt dispersed and impregnated as a catalytically activelayer upon the surface of a support containing at least about 80 wt %titania ranging in average thickness from about 0.02 mm to about 0.20mm, with a cobalt loading of about 0.04 g/cc to about 0.15 g/cc,calculated as metallic cobalt per packed bulk volume of catalyst andwith a productivity and methane selectivity at 200° C. of at least 150hr⁻¹ and no more than 10 mole %, respectively.
 2. The process of claim 1wherein the molar ratio of H₂ :CO ranges from about 0.5:1 to about 4:1.3. The process of claim 1 wherein the molar ratio of H₂ :CO ranges fromabout 1.7:1 to about 2.5:1.
 4. The process of claim 1 wherein the totalpressure of the reaction ranges from about 80 psig to about 600 psig. 5.The process of claim 4 wherein the total pressure of the reaction rangesfrom about 140 psig to about 400 psig.
 6. The process of claim 1 whereinthe catalytically active surface layer of the catalyst is of averagethickness ranging from about 0.04 mm to about 0.20 mm.
 7. The process ofclaim 6 wherein rhenium constitutes part of the catalytically activesurface layer of the catalyst and the weight ratio of cobalt to rheniumranges from about 30:1 to 2:1.
 8. The process of claim 6 wherein hafniumconstitutes part of the catalytically active surface layer of thecatalyst and the weight ratio of cobalt to hafnium ranges from about30:1 to 2:1.
 9. The process of claim 1 wherein the titania has arutile:anatase weight of at least 3:2.
 10. The process of claim 1wherein the titania has a rutile:anatase weight ratio ranging at leastabout 3:2 to about 100:1, and higher.
 11. The process of claim 10wherein the rutile:anatase ratio ranges from about 4:1 to about 100:1,and higher.